These studies are designed to define the molecular lesions affecting Beta globin production in patients with homozygous Beta thalassemia. Earlier work performed in this and other laboratories have suggested that Beta thalassemia is frequently caused by mutations which affect the normal pattern of processing of Beta globing mRNA precursors. During the past year, we have attempted to define defects in RNA processing which occur in bone marrow cells of patients with Beta thalassemia, and to clone genes from patients who have been identified as having novel RNA processing defects. Three types of abnormal RNA molecules have been demonstrated: The first two reflect defects which lead to abnormal processing of the smaller intervening sequence of the Beta globin gene transcript. The third is present in a patient who preferentially uses an upstream initiation site of RNA transcription which is infrequently used in normal bone marrow cells. Two thalassemic globin genes have been clonedm sequenced, abd studied in a eukaryotic expression system. One of these genes contains a single base pair mutation in the first coding block of the Beta gene, resulting in an abnormal splicing pattern. The second gene contains a previously described mutation which prematurely stops translation of processed Beta mRNA. Transcripts from both of these abnormal genes appear to be relatively unstable unstable in a eukaryotic expression system.